Amplifiers of the type under consideration are used for the amplification of electrical oscillations generated by means of a musical instrument. In order to obtain the sound waves required for a concert, it is possible to use a plurality of loudspeakers disposed downstream of an amplifier. Another possibility is to use an amplifying and loudspeaker system, either already in place or to be installed in a concert hall, for increasing output and reproduction. In this case a preamplifier, disposed downstream of the instrument, is generally used to adjust the musical sound or to obtain certain sound effects. If operated in the way last mentioned, a problem occurs in that the particular timbre resulting from the combination of, in particular, tube amplifiers with loudspeakers disposed immediately downstream of them, cannot be achieved. This combination is something which is considered to be especially desirable by concert artists.